Until last weeks release of Windows 10 Mobile for Current Branch (Build 10586.164), the only handsets that were allowed to participate in the Windows Insider Program for the upcoming Redstone release were those off retail shelves.
That included just six handsets including the Lumia 550, 650, 950, 950 XL, Alcatel OneTouch Fierce XL and Xiaomi Mi4.
With the release of Windows 10 Mobile to some existing Windows Phone 8.1 handsets that means eligibility for testing Redstone builds of Windows 10 Mobile opened up to 18 new handsets.
As I have monitored social media since the Windows 10 Mobile CB release last Thursday, there have been a lot of questions about when owners of those 18 devices can get those new builds.
The answer to that question is actually not in any of the official blog posts or information pages for Windows 10 Mobile.
It is located in two locations - the Windows Insider Hub or in a post from the Windows Team on the Microsoft Answers Community Forums.
That entry in the Answers Forum, which was written by Jason from MS aka @NorthFaceHiker on Twitter, contains a quote of that Insider Hub entry from Gabe Aul and it includes this sentence (emphasis mine):
"As we stated previously, devices which are eligible to receive Windows 10 will also be able to get preview builds from the Development branch. We’ve already delivered a few builds to the devices which shipped with Windows 10, including the Lumia 550, 650, 950, 950 XL, Xiaomi Mi4, and Alcatel OneTouch Fierce XL. We have updated the list of devices that will be capable of receiving updates as part of the Windows Insider Program going forward. Devices on this list that opt in to the Fast or Slow rings, will begin to receive new Windows 10 Insider Preview builds from the Development Branch in the coming weeks."
In another spot they indicate those handsets might not see Redstone builds until a future build (emphasis mine again)
"Currently builds from the development branch have been made available to the Lumia 950, 950 XL, 650, 550, Xiaomi Mi4, and Alcatel OneTouch Fierce XL. In the coming weeks we will be making future Windows 10 Insider Preview builds available to devices on the updated Windows 10 Mobile device list. We will add devices to the Windows Insider Program as more devices launch with Windows 10 Mobile."
This means you can take a couple of steps to be ready but otherwise it is time to wait.
To get started if you are using one of those 18 handsets that were added to the official Windows 10 eligible list install the Windows Insider app and opt into the Fast Ring so that when your handset is provisioned for Redstone you will get an update for that build.
There are no Slow Ring releases for Redstone so the only option to get the latest build in the development branch is to enroll your device in the Fast Ring.
Of course, Fast Ring means more bugs and more known issues so user beware.